With the advance of wireless communication technology, plenty of convenient wireless communication systems have been developed. These systems include global system for mobile communication (GSM), personal communication service (PCS), and wireless local area network (WLAN), etc. The radio-frequency (RF) modules adopted in conventional single-band systems are not sufficient for current wireless communication systems that essentially emphasize multiple functions. The multi-band or even multi-mode modules have become the future trend of the RF modules.
The structures of a matching circuit (e.g., inductors or transmission lines) associated with filters of different frequency bands are similar to the duplexer designs disclosed in U.S. Pat. Nos. 6,707,350, 6,414,567, and 6,411,178. In U.S. Pat. No. 6,707,350, the band-pass filter of a duplexer uses a direct input structure. The band-pass filter structure of a duplexer disclosed in U.S. Pat. No. 6,414,567 consists of three resonator These resonators are coupled through capacitive coupling, and inductors are used for the design of the matching circuit. In another U.S. Pat. No. 6,411,178, the band-pass filter structure of a disclosed duplexer comprises three resonators, and its matching circuit adopts a serial combination of capacitors and inductors.
The major function of a triplexer is to separate a received signal into different frequency bands with good isolation. Conventional triplexers are designed with low-pass and high-pass filters or plural band-pass filters. The former design has the advantage of low insertion loss and good isolation but its drawback is a large distortion outside the allowed frequency band. The latter design has the advantage of good selectivity among various frequency bands but its design is quite complicated. The complexity of the design results from a requirement of many stages for band-pass filters. Furthermore, it has a high insertion loss.